Thanks for working on this! I mainly have two feedbacks other than those inline comments:

1. All Java request and reposes classes inherit from RESTMessage below with a validate method. I think this is important because these objects may be created out of our control and must be validated before use.

/** Interface to mark both REST requests and responses. */
public interface RESTMessage {

  /**
   * Ensures that a constructed instance of a REST message is valid according to the REST spec.
   *
   * <p>This is needed when parsing data that comes from external sources and the object might have
   * been constructed without all the required fields present.
   */
  void validate();
}

I'd propose to add a iceberg/catalog/rest/validator with interface like below for every request and response:

struct Validator {
  static Status Validate(const ListNamespacesResponse& resp);
  ...
};

2. Interoperability is our top priority. I have followed all Java test cases for TableMetadata serde by adding them to src/iceberg/test/metadata_serde_test.cc. There are a lot of test in the Java repo like org/apache/iceberg/rest/responses/TestListNamespacesResponse.java which contains both good and bad cases. It would be good to include them while implementing each objects.

Great job!